144. The number at the end of the times table, the merciless grid emblazoned on countless classroom walls for so many years, and no doubt in those of the Pantglas Junior School in Aberfan, South Wales, whose obliteration by a colliery slagheap landslide 40 years ago this week has been shamefully all but overlooked by TV. Thank God, then, for this unassuming yet quietly impressive documentary, squirreled away by ITV1 beyond the watershed.

144. It has resonance for any junior schooler. Yes – that number at the end of the times table, and the number of people that died when a school and half a village was engulfed. This reviewer was scarcely even there yet, scarcely alive when all those kids died. But I remember it, and always will. 144.

The show revolved around the return of the old Time magazine photographer, Chuck Rapoport, to the village from whence his neo-realist photographs of grief and desolation (Brandt, Doisneau) had so shocked the world. Aberfan is, Rapoport found, much the same. The disquieting TV footage of the rescue (as the commentary pointed out, it was one of television’s first major disasters), with its antlike hordes and human chains almost a blackly comical resistance to the mountain of debris, looks as distant as Battleship Potemkin. In two of the snapper’s best shots, a podgy little survivor, traumatised beyond imagining, in duffel and NHS specs, uncertainly fringes a shot of frowning tips and slates and heathery mountains. In another, the sister of a bereaved mother clutches a soaked brolly as though it will stave off further disaster, further godly retribution.

Even now, watching Rapoport return, it’s hard to tell if the pictures are in monochrome or colour; the bricks are often the colour of liver or arterial blood. From a distance, the cemetery where the bodies lie looks like little more than a jumble of fallen and splintered sarsens. Everywhere, everything, is the colour of cheap dull clothing.

In 1966, Aberfan was definitely not swinging. It never was, it never will. There was no Biba here, no Floyd, no Play Power. The haircuts in the archive footage date from the 1920s, not Carnaby Street. There was as little license in Aberfan as there was oxygen for the drowned kids to breathe.

But how can it be otherwise? The Welsh valleys are full of apocalyptics, Baptist freaks, ascetics; John Collins, famously and unimaginably, lost his entire family while out at work. Wife, kids, house, extinguished by the black flood, a Job-like Biblical smiting of a kind the preachers used to promise to the unfaithful in pulpits from Brynmawr to Llanelli. Maybe the most monstrous irony of all was that a mining village should suffer its greatest tragedy above ground.

This was all tactfully, if untelegenically, recorded (Aberfan doesn’t do pretty – but then again, neither do mine catastrophes). The mayor of Merthyr Tydfil breaks down when a six-year-old moppet asks him, “Were you sad when your friends died?” Another survivor displays his unfeasibly mangled right hand and his scars. Did he fight the tears as much that day as when he told the camera, “I was shouting for my mam, like?” in between long breaths that issue from him like a very old puncture. One other lucky child, now as unhealthily plumptious of girth as most valleys residents, is pictured holding a Subbuteo set that Rapoport’s journalist chum gave him as a present in 1966.

Cello, as well as sparse, plashy modal piano (“file under moody”) features predictably on a soundtrack which is laudably and decently minimal. Wider issues were skirted; negligence on behalf of the NCB, and the geological reasons for the landslide, were simply ignored. Plus, the quality of the whole and in particular its production values, was compromised, wan, make-do – but it works perfectly for such a down-at-heel town, and the immeasurable pathos it embodies.

It also stresses the importance of simplicity in TV’s discourse of the most interesting and absorbing of history lessons, those told by its makers or victims. It is the sort of modestly impressive documentary TV that most ITV and BBC regions used to make and relegate to the graveyard slots – which, with a decent budget and genuine commitment, could make it to prime time and anthology tapes for ever.

ITV1 seem to feel the same, which sadly diminishes the considerable honour due to them for having Children of the Valley made in the first place.

This one’s in my library already – it should be in yours.